Photovoltaic cells often have a structure including a stack of thin films in which at least one of said thin films has photovoltaic properties.
An example of a structure of photovoltaic cells is shown in FIG. 1.
As shown in FIG. 1, a photovoltaic cell generally has a stack of films placed on an insulating support 12. In general, the insulating support 12 is a layer of glass.
A film of molybdenum 14 having a thickness of between 0.5 μm and 1 μm is deposited on the insulating support 12. The molybdenum film 14 is generally deposited by vacuum evaporation or sputtering.
An absorber film 16 is deposited on the molybdenum film 14. The absorber film 16 generally has a thickness of about 2 microns and may be deposited by vacuum evaporation, or by cathode sputtering.
An interfacial film 18 is deposited on the absorber film 16. The interfacial film 18, also called buffer film, may comprise cadmium sulfide or zinc sulfide deposited chemically in solution. The interfacial film 18 generally has a thickness of between 10 nm and 80 nm.
A slightly doped zinc oxide film 20 is deposited by cathode sputtering on the interfacial film 18. The slightly doped zinc oxide film 20 has a thickness of about 50 nm to 100 nm.
An aluminum-doped zinc oxide film 22 is deposited on the slightly doped zinc oxide film 20. The aluminum-doped zinc oxide film 22 is conventionally deposited by vacuum sputtering in order to have a thickness of about 0.5 μm to 1 μm.
The doping of the said zinc oxide film is intended to make said film n-type conductive and to serve as an electrode (transparent in the visible) at the front of the photovoltaic cell.
Photovoltaic cells in which the absorber film 16 consists of CuInSe2 or Cu(Inx,Ga1-x)Se2 compounds have conversion efficiencies of up to 20%.
The principal method for preparing photovoltaic cells in thin films makes use of physical methods, for example co-evaporation or even cathode sputtering.
The photovoltaic properties of the CuInSe2 or Cu(Inx,Ga1-x)Se2 thin films are considerably dependent on the composition of the absorber thin film. Thus, it is important to be able to control the composition of the absorber thin film as accurately as possible.
Electrodeposition is a method which can serve to improve the control of the composition of the CuInSe2 or Cu(Inx,Ga1-x)Se2 thin films.
A method for electrodepositing a CuInSe2 alloy is described in application U.S. Pat. No. 4,581,108.
The method described in application U.S. Pat. No. 4,581,108 comprises the following successive steps:                electrodepositing a thin film of copper (Cu) and indium (In),        addition of selenium (Se) by selenization in order to form a CuInSe2 film.        
The preparation of an absorber thin film by electrodeposition is complicated to implement, inter alia, because of:                the very wide difference in redox potentials of the various constituent elements of the thin film,        the low solubility of indium and/or gallium salts,        the strong affinity of gallium for oxygen, and        the great complexity of selenium chemistry.        
A need therefore exists for an easily applicable method for producing absorber films having a well controlled composition.